Word had spread that a prisoner swap was imminent in recent days as Sentsov was moved to Moscow from a prison in Russia’s Arctic north.

The Ukrainian was arrested in 2014 during a protest against Russia’s annexation of Crimea, a Ukrainian peninsula,

Sentsov was given a 20-year jail sentence after prosecutors said he was involved in two attempted arson attacks in the city of Simferopol, ordered by extremist Ukrainian group Right Sector. He denied the charges.

The trial was condemned by the US, EU and other film directors.

Sentsov started a hunger strike in May last year to press Russia to free 64 jailed Ukrainians whom he calls political prisoners. After more than 140 days he abandoned the strike to avoid being force fed.

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Media captionFootage of the Russia-Ukraine sea clash

The sailors were seized along with three naval ships as they attempted to pass through the Kerch Strait, the only access to Ukrainian ports on the Sea of Azov.